From: Jonny Grant <jg@jguk.org>
To: Jonathan Wakely <jwakely.gcc@gmail.com>
Cc: Xi Ruoyao <xry111@xry111.site>, gcc-help <gcc-help@gcc.gnu.org>
Subject: Re: std::string add nullptr attribute
Date: Mon, 13 Feb 2023 22:02:38 +0000 [thread overview]
Message-ID: <09fc5748-53f2-5e81-80df-c617800a2b05@jguk.org> (raw)
In-Reply-To: <CAH6eHdRocyiTmhs_WKg1n2zewecM6isQFdiLU5b8SbQ8_sYGfA@mail.gmail.com>
On 11/02/2023 00:32, Jonathan Wakely wrote:
> On Fri, 10 Feb 2023 at 22:38, Jonny Grant <jg@jguk.org> wrote:
>>
>>
>>
>> On 10/02/2023 22:03, Jonathan Wakely wrote:
>>> On Fri, 10 Feb 2023 at 21:30, Jonny Grant <jg@jguk.org> wrote:
>>>>
>>>>
>>>>
>>>> On 09/02/2023 17:52, Jonathan Wakely wrote:
>>>>> On Thu, 9 Feb 2023 at 16:30, Xi Ruoyao wrote:
>>>>>>
>>>>>> On Thu, 2023-02-09 at 14:56 +0000, Jonathan Wakely via Gcc-help wrote:
>>>>>>>> Note, my code isn't like this, it is just an example to suggest
>>>>>>>> adding the nullptr attribute, as its clearly already rejected at
>>>>>>>> runtime.
>>>>>>>
>>>>>>> I assume you mean the nonnull attribute. That was added in 2020 and
>>>>>>> then reverted because it broke some things:
>>>>>>
>>>>>> I remember I'd once made the same mistake when I suggested to add
>>>>>> nonnull for ostream::operator<<(const string &) and I was lectured:
>>>>>> nonnull is not only a diagnostic attribute, it also allows the compiler
>>>>>> to assume the parameter is never null and rendering std::string(nullptr)
>>>>>> an undefined behavior.
>>>>>
>>>>> Yes, I think that's what might have happened with the std::string change.
>>>>
>>>> My apologies, Jonathan, Xi, yes it is the __attribute__((nonnull)); I was mistaken to type as nullptr.
>>>>
>>>> I re-read, and it does seem nonnull is really an optimization that as a side effect may give some warnings. So I'm going to stop using it.
>>>> https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#Common-Function-Attributes
>>>>
>>>> (there is a typo in that manual section saying "nonnul" - I don't know if you have a moment to make a change in git? I didn't get replies on gcc-patches to my patches...)
>>>>
>>>> I searched and see like someone investigated this problem and saw it removed NULL checks http://www.rkoucha.fr/tech_corner/nonnull_gcc_attribute.html
>>>>
>>>> I saw wget2 removed the nonnull attribute due to the optimizer removing checks against NULL too
>>>> https://gitlab.com/gnuwget/wget2/-/issues/200
>>>>
>>>>>> Then the example may just silently continue to run, instead of throwing
>>>>>> an exception. It would be an ironic example: an attempt to improve
>>>>>> diagnostic finally made diagnostic more difficult.
>>>>>
>>>>> Indeed.
>>>>>
>>>>> Maybe we can add __attribute__((access(read, 1))) instead, which says
>>>>> that we will read from the pointer, which also implies it must be
>>>>> non-null.
>>>>
>>>> I tried this with gcc 12, as read_only, but it didn't stop when compiling. Maybe you have an example that demonstrates please?
>>>>
>>>> void f(const char * p) __attribute__((access(read_only, 1)));
>>>>
>>>>>
>>>>> N.B. in C++23 string(nullptr) produces an error, although
>>>>> string((const char*)nullptr) doesn't, so in practice it only prevents
>>>>> the dumbest calls with a literal 'nullptr' token, and not the more
>>>>> realistic problems where you have a pointer that happens to be null.
>>>>
>>>> That's good it stops compiling, the error is not that clear "use of deleted function" for me though.
>>>>
>>>> string.cpp: In function ‘int main()’:
>>>> string.cpp:13:26: error: use of deleted function ‘std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(std::nullptr_t) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; std::nullptr_t = std::nullptr_t]’
>>>> 13 | std::string c(nullptr);
>>>>
>>>>
>>>>
>>>>
>>>> I made my own test class str_string which stops the build a different way. It only works if the dumbest calls with 'nullptr' as you found in your test.
>>>>
>>>> void nullptr_compile_abort() __attribute__((error("nullptr compile error")));
>>>>
>>>> str_string(nullptr_t) { nullptr_compile_abort(); }
>>>
>>> This doesn't work because std::is_constructible_v<std::string,
>>> std::nullptr_t> would be true, and we want it to be false.
>>
>> Hmm, for me, this output is 0.
>> std::cout << std::is_constructible_v<std::string,std::nullptr_t> << "\n";
>
> For C++23, yes, but if you add a constructor like your
> str_string(nullptr_t) it would become 1.
>
> Using a deleted function is observably different to using a
> constructor that then produces an error when called.
Indeed.
May I ask if you found a way to get the read_only attribute to trigger a build warning for nullptr?
It seems like only runtime checks can catch most of them for the moment.
Jonny
next prev parent reply other threads:[~2023-02-13 22:02 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-02-09 13:26 Jonny Grant
2023-02-09 14:56 ` Jonathan Wakely
2023-02-09 16:30 ` Xi Ruoyao
2023-02-09 17:52 ` Jonathan Wakely
2023-02-10 21:30 ` Jonny Grant
2023-02-10 22:03 ` Jonathan Wakely
2023-02-10 22:38 ` Jonny Grant
2023-02-11 0:32 ` Jonathan Wakely
2023-02-13 22:02 ` Jonny Grant [this message]
2023-02-19 20:43 ` Jonny Grant
2023-02-19 21:33 ` Jonny Grant
2023-02-20 10:26 ` Xi Ruoyao
2023-02-20 10:37 ` Jonathan Wakely
2023-02-20 10:54 ` Xi Ruoyao
2023-02-20 11:10 ` Gabriel Ravier
2023-02-20 11:18 ` Marc Glisse
2023-02-20 11:28 ` Segher Boessenkool
2023-02-20 12:00 ` Jonny Grant
2023-02-20 14:50 ` Gabriel Ravier
2023-02-20 11:44 ` Jonny Grant
2023-02-21 15:02 ` Jonny Grant
2023-02-20 11:38 ` Jonny Grant
2023-02-20 11:30 ` Jonny Grant
2023-02-20 12:59 ` Xi Ruoyao
2023-02-20 13:44 ` Jonathan Wakely
2023-02-20 19:21 ` Jonny Grant
2023-02-20 19:35 ` Jonathan Wakely
2023-02-20 19:39 ` Jonny Grant
2023-02-22 20:27 ` Jonny Grant
2023-02-21 15:04 ` Jonny Grant
2023-02-21 22:48 ` Jonny Grant
2023-03-04 15:00 ` Jonny Grant
2023-02-20 11:25 ` Jonny Grant
2023-03-12 22:10 ` Jonny Grant
2023-03-13 10:10 ` Jonathan Wakely
2023-03-13 19:55 ` Jonny Grant
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